Messier 75

Messier 75 or M75, also known as NGC 6864, is a giant globular cluster of stars in the southern constellation Sagittarius. It was discovered by Pierre Méchain in 1780 and included in Charles Messier's catalog of comet-like objects that same year.

Messier 75
Observation data (J2000 epoch)
Class	I
Constellation	Sagittarius
Right ascension	20h 06m 04.85s
Declination	−21° 55′ 17.85″
Distance	68 kly (20.9 kpc)
Apparent magnitude (V)	8.5
Apparent dimensions (V)	6.8′
Physical characteristics
Absolute magnitude	−8.57
Radius	67 ly
Tidal radius	5.7′
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = −1.29 dex
Other designations	GCl 116, M75, NGC 6864

M75 is about 67,500 light years away from Earth and is 14,700 light years away from, and on the opposite side of, the Galactic Center. Its apparent size on the sky translates to a true radius of 67 light years. M75 is classified as class I, meaning it is one of the more densely concentrated globular clusters known. It shows a slow rotation around an axis that is inclined along a position angle of −15°±30°. The absolute magnitude of M75 is about −8.5, equating to 180,000 times more luminous than the Sun (L_☉).

The cluster has a half-light radius of 9.1 ly (2.80 pc) with a core radius of about 1.6 ly (0.5 pc) and appears not to have undergone core collapse yet. The mass density at the core is 7.9×10⁴ M_☉·pc⁻³. There are 38 RR Lyrae variable stars and the cluster appears to be Oosterhoff-intermediate in terms of metallicity. 62 candidate blue stragglers have been identified in the cluster field, with 60% being in the core region.

Messier 75 is part of the Gaia Sausage, the hypothesized remains of a dwarf galaxy that merged with the Milky Way. It is a halo object with an orbital period of 0.4 billion years to travel around the galaxy on a very pronounced ellipse, specifically eccentricity of 0.87. The apocenter (maximal distance from Earth) is about 57,000 ly (17,500 pc).